Right now it spits back HTML but it is trivial to also add JSON support.

I was speaking to a developer who's been using the API for awhile, and he mentioned once it's picked up, even if it's sitting in a queue, it can't be cancelled. Is it possible to get the agent to poll for cancelled tests before running the test, or remove it from the queue (maybe by polling or removing it from the list)?

I just tested to be sure and the canceling looks to be working fine. Are you trying to cancel a test that was run through a relay? If so that isn't currently supported. Tests managed by the local server should cancel fine as long as they are still in the queue (I just tested it on the public instance to be sure: http://www.webpagetest.org/result/130719...03e5fa095/ )